Acoustical device



Sept. 22, 1931. A P. MONTGOMERY 1,824,730

ACOUSTICAL DEVICE Filed April 30, 1930 BY P.

' ATTORNEY Patented Sept. 22, 1931 UNITED S A ES;

' .Annnnw nMonrGoMEny, or KEW eARnEivs, N YORK, ASSIGNOB- 'ro Hickman a 00., 11m, onnroHMoNn HILL, NEW YoamAooiaroRArIon oEnEw YORK PATENT OFFICE" ,ACOUSTICAL nEvIoE' Application filed April 30,

My invention relates to the improvement of sound amplifiers or loud speakers and while not limited in its application, isspecially adapted for use in electro-dynamic de-' vices which operate to producesound waves by the interactionof a movable coil carrying a varying electric currentyanda. magnetic field produced by a coil carrying direct cur-- rent in which there is some fluctuation.

Several constructions suitable for carry ing my invention into'efiect, are illustrated in the accompanyingdrawings', but I do not a wish to be understood as limiting myself to either the exact form oredet'ailsshown as 15 both may be modified without departing from the spirit and scope of the present invention. In Fig. 1 of the drawings. there is shown:

a simplified cross sectional diagram ofonej form of the above described electro-dynamicr device, in which there is also shown incor-j porated one form of. my invention for im q proving the operation of the same; 1 .7

p In Figs. 2, 3 and 4, I haveshown other forms in which my invention may be applied to get somewhat better results than is obtained by the simple form of Fig. 1.

Referring now to the drawings, the-magnetic structure of the electro-dynamic device is shown to consist of a' core A, fastened centrally within a surrounding h0llow-cy-- lindrieal member B, headed at'both ends and; provided in one head with an aperture 6', through which the core A projects. The relative diameters of the aperture 5 and'thei core A, are such as to provide a narrow annular air gap, shown at-C." In this air gap there is supported in suitable manner a small A q coil D, which is so constrained astobe able 40' to move in the air gap axially, while holding its central alignment. 1

The core A, carries the usual windingE, which fills the hollow cylindrical member to p the greater portion 'of its depth,'leavin'g a space above the winding which is-ordinarilyfilled by a heavy ring or diskof copper, the use of which is objectionable for reasons to be hereinafter explained. 1 T

Currents which carry the musicor voice to be reproduced are sentthrou'gh; coil 1930. Serial panama.

D,-and it moves back and forth axiallyrdue to the interactionof the current and the radial. .magnetic field in which the coil'is lying. A diaphragm of some sort, indicated here as v I generally made of many turns of Wire. f The character, such as is delivered by a good battery, in which case the magnetic field in current in this 'coil E may be of uniform the air gap is uniform in intensity, not varying with time. In manyv cases, however,.the cur-rentthrough-coil E, from other sources of power, is of a pulsatin character, and in 4 such cases the magnetic V eld in the airgap C is. also pulsating, increasing and decreasmg in intensity according to the ,current through coil E.

Now,fthe coil D isgenerally. connected to.- the secondary circuit of a so-called output transformer so designed as tomake the impedance of coil D match this circuit which is furnishing the electric power to operategthe device. Whether or not this is the case, the; coilfD forms part of a closed electric circuit, a circuit in which current will flowif an. electro-active force isset up in the circuit;

If the magnetic field in the air gap C is of variable intensity,- a voltage will be set. up

in coil D, according to the well known laws.

'ofinduced ele'ctro-motive force. That is, if

thismagnetic field'in the airgap waxes and wanes, say 120 times'a second, an alternating voltage of that frequency will'be set up 1n coil-D, and this alternating voltage will cause alternating current to flow in'the circuit of which coilD is apart. I Now, this alternating current in coil Dwlll cause it to vibrate axially, thus making the diaphragm send off corresponding sound waves. This is the cause of the'famihar hum prevalent toagreater or less extent in practically all loud speakers of this type furnished with the modern radio receiving set. Even with no signal cominginto the speaker from the receiving circuit, the speaker generates a hum, due to the non-uniformity of the current flowingthrough coil 'E.

Coil E is generally used as a choke coil in the modern radio receiver as serving to help suppress the ripples of current in the rectifled power supply for the plate circuits of the receiver. But while performing this function, the coil does generate fluctuations in the magnetic field in the air gap, and this causes the hum previously mentioned.

One of the principal objects of the present, invention is the prevention of thishum in. devices of the type shown and'similarly opnetizing coil E.

Previous attempts to accomplish this result have utilized rings of copper, or other conducting material, surrounding the pole piece A, and placed between the end of coil E and the air gap DJ Such attempts have notproven entirely satisfactoryin spiteof the fact that large amounts of copper or aluminum have been employed, thus considerably increasing the cost of the device. The increased cost is due not only to the amount'of the material used in the rings'themselves, but to the increased size of the device as a whole, to accommodate the rings. I My invention consists of associating with such rings of conducting material, considerably reduced in thickness, one or more rings, or other conveniently formed pieces, of magnetic material. I have found a remarkable diminution in the hum of the loud speaker with the use of onlyone such added ring of iron, placed about as shown at G, Fig. 1. In this figure, the associated ring ofconducting material is shown at H.

The ring H I term a damping ring, because its effect is to damp out, or diminish, the flue tuations in the magnetic'flux of core A, where this comes in proximity to the air gap. In the core A, inside of coil E, there must necessarily be-fluctuations in the magnetic flux, if core E is to carry pulsating current and is to act as an effective choke coil. 'My invention there serves to prevent these fluctuations in magneticflux from reaching'the air gap C. The fluctuations set up currents in the ring H, Fig. 1, and thesecurrents, tosome extent, act to keep the flux in the air gap constant,

but my invention greatly increases the efiec-' tiveness of their action.

- While I do not limit myself in any way by my explanation of the action of my invention, it seems likely that the greatly increased efiectiveness of the damping ring H 1s due to the increase in its reactance. by the action of the closely adjacent magnetic ring G. To effectively damp out the fluctuations in the air gap flux, the currents which are induced the damping ring H should be 90 degrees phase, behind the voltage induced in this rlng, and the effect of the magnetic ring H is greatly enhanced by the addition of the magnetic ring G. So reat is the eflfect that a greatly diminished thickness of damping ring, in combination with only one of my magnetic rings, results in a great diminution in hum of the loudspeaker.

' Ihave'furthermore discovered that if more than one conductin ring H and one magnetic ring G (Fig. 1 are used, the effectiveness of the scheme is further increased. Thus, in Fig. 2, I have shown a magnetic ring G, G, on either side of the conducting ring H. In Fig. 3, I have shown two conducting rings H, H, and two magnetic rings'G, G, and in Fig. 4, I have shown a still more laminated structure. 'My measurements show that as the number of-conducting rings, and correspondingly the number of interspersed magnetic rings, is increased, the effectiveness of a given amount of-material is correspondingly increased. i

Whereas, I do not wish to limit this applicationof my idea'tocertain specific materials, I have found it preferable to use either copper or aluminium for the conducting rings and mild steel for the magnetic rings. Neither do I limit myself to the exact proportions of dimensions as shown in my drawings; it maysornetimes be advantageous to have the-magnetic rings of greater diameter than the conducting rings, and with some designsof magnetic structure, the magnetic rings may be larger or smaller in diameter, compared to the dimensions of the magnetic circuit as a whole than is indicatedin my drawings.

Having thus described my invention, what I claim is:

1. In an electro-magnetic sound reproducer employing a magnetizing coil carrying a pulsating magnetizing current,'an air gap in which a pulsating magnetic field is developed, means for diminishing the amount of pulsation of said field, said means consisting of a laminated ring placed between the air gap and the magnetizing coil, said laminated ring consisting of alternating laminae of magnetic material and non-magnetic material of high conductivity.

2. Ina loud speaker of the electro-dynamic type, having a magnetic core, a magnetizing coil carrying pulsating current and an air gap carrying a pulsating flux set up by the pulsating magnetizing current, means to diminish the pulsations in air gap flux, said means consisting of a laminated ring placed between the air gap and the magnetizing coil. said laminated ring consisting of alternating laminae of magnetic material and non-magnetic material of high conductivity. a; In a loudspeaker of the'electro-dynamic type, having a central core magnetized by an exciting coil carrying pulsating current and an air gap in which a pulsating magnetic field is developed, means to diminish the pulsations in said magnetic field, said means consisting of a laminated ring placed between the air gap and the magnetizing coil,

said laminated ring consisting of alternating laminae of magnetic material and non-magnetic material of high conductivity.

4. In a loud speaker of the electro-dynamic type, having a core magnetized by an exciting coil carrying pulsating current, an air gap around the core in which a pulsating magnetic field is generated, and means to diminish the pulsations in the air gap flux, said means consisting of copper and iron members placed about the core between the exciting coil and the air gap.

5. In a loud speaker of the electro-dynamic type, having a core magnetized by an exciting coil carrying pulsating current, an air gap around the core in which a pulsating magnetic field is generated, and means to diminish the pulsations in the air gap flux, said means consisting of copper and iron rings arranged in alternating relation about the core between the exciting coil and the air gap.

6. In a loud speaker of the electro-dynamic type, having a core magnetized by an exciting coil carrying pulsating current, an air gap around the core in which a pulsating magnetic field is generated,and means to diminish the pulsations in the air gap flux,

said means consisting of conducting members and magnetic members arranged in alternating relation around the corebetween the exciting coil and the air gap.

7. In a loud speaker of the electro-dynamic type, having a core magnetized by an exciting coil carrying pulsating current, an air gap around the core in which a pulsating magnetic field is generated, and means including a magnetic member interposed between the exciting coil and the air gap acting to diminish the pulsations in the air gap flux.

I aflix my signature.

ANDREW P. MONTGOMERY. 

